There are many instances when it is required to separate one compound, such as a contaminant or a desired molecule, from a liquid. Charge-charge based interactions are used in a number of fields to capture and hence separate charged or chargeable compounds.
In the chemical and biotech field, target compounds such as drug or drug candidates usually need to be separated from contaminating species originating from the process of manufacture. For example, a protein drug or drug candidate produced by expression of recombinant host cells will need to be separated e.g. from the host cells and possibly cell debris, other host cell proteins, DNA, RNA, and any other compounds originating from the fermentation or cell culture broth. Due to its versatility and sensitivity to the target compounds, chromatography is involved as at least one step in many of the currently used biotech purification schemes. The term chromatography embraces a family of closely related separation methods, which are all based on the principle that two mutually immiscible phases are brought into contact. More specifically, the target compound is introduced into a mobile phase, which is contacted with a stationary phase. The target compound will then undergo a series of interactions between the stationary and mobile phases as it is being carried through the system by the mobile phase. The interactions exploit differences in the physical or chemical properties of the components of the sample.
The stationary phase in chromatography is comprised of a solid carrier to which ligands, which are functional groups capable of interaction with the target compound, have been coupled. Consequently, the ligands will impart to the carrier the ability to effect the separation, identification, and/or purification of molecules of interest. Liquid chromatography methods are commonly named after the interaction principle utilized to separate compounds. For example, ion exchange chromatography is based on charge-charge interactions; hydrophobic interaction chromatography (HIC) utilizes hydrophobic interactions; and affinity chromatography is based on specific biological affinities.
As is well known, ion exchange is based on the reversible interaction between a charged target compound and an oppositely charged chromatography matrix. The elution is most commonly performed by increasing the salt concentration, but changing the pH is equally possible. Ion-exchangers are divided into cation-exchangers, wherein a negatively charged chromatography matrix is used to adsorb a positively charged target compound; and anion-exchangers, wherein a positively charged chromatography matrix is used to adsorb a negatively charged target compound. The term “strong” ion exchanger is used for an ion-exchanger which is charged over broad pH intervals, while a “weak” ion-exchanger is chargeable at certain pH values. One commonly used strong cation-exchanger comprises sulphonate ligands, known as S groups. In some cases, such cation exchangers are named by the group formed by the functional group and its linker to the carrier; for example SP cation exchangers wherein the S groups are linked by propyl to the carrier.
The charged groups in ion exchangers (often called ligands) can be attached to the carriers or support materials in different ways. Several publications (U.S. Pat. No. 5,453,186, WO2008145270, U.S. Pat. No. 8,092,682, WO2012015379, EP2412433 and EP2412435) describe how charged monomers can be graft polymerized on support materials to form ion exchangers where the ligands are present on pendant graft polymer chains covalently attached to the supports. However, in particular in the bioprocessing area, the demands on the separation matrices are continuously increasing and there is hence a need for further developments, particularly with respect to matrices providing improved selectivity and capacity, as well as stability during alkaline cleaning.